Implantable pacemakers and other implantable stimulus devices must be designed for efficient generation and delivery of stimulus pulses. Such stimulus pulse generation must meet a number of criteria. The generated pulse must be controllable in terms of energy level and other parameters. Typically for pacemakers, the energy level is programmable or adjustable with increments of 0.1 volts between about 0.5 V and 1.5 or 2 V, and within a duration of about 0.1-1.5 ms, so that the pulse energy level can be adjusted with respect to the patient pacing threshold. Further, as discussed in U.S. Pat. No. 4,343,312, which is incorporated herein by reference, it is desirable to deliver a stimulus which minimizes the polarization that results at the electrode where the pulse is delivered. In the pacemaker environment, minimization of such polarization is highly desirable in order to enable enhanced detection of evoked responses, and better detection of other important signal information such as the T-wave. The aforementioned U.S. Pat. No. 4,343,312 discloses a pulse generator for generating a triphasic pulse, comprising positive recharge pulses immediately before and after the negative, or discharging stimulus signal, the recharge pulses being adapted in time duration and amplitude such that the total current delivered at the electrode is substantially zero, thereby minimizing polarization.
Another important feature of a pulse generator output stage is minimization of capacitors. In a typical output stage, such as the referenced triphasic stage, a nominally large holding capacitor of up to about 20 .mu.F is required in order to hold the desired voltage value which has been obtained by raising the battery voltage through a charge pump. Such a large capacitor not only adds to cost, but presents a larger size which is crucially important in an implantable device such a pacemaker.
Another problem with prior art circuits is that of adjusting the pacing voltage in desirably small voltage steps. A typical prior art pump circuit as found in pacemakers provides for a plurality of different capacitor configurations, each yielding an output voltage which is a different multiple of the battery voltage. However, there is a limit to the number of capacitors that can be utilized for obtaining different voltage values, for cost and space reasons.
Accordingly, there remains a need in the art for an improved output stage for battery-driven devices such as pacemakers, neural stimulators, and the like, which can provide better control of the output pulse parameters and be realized with savings in required capacitors.